Ultra-thin glass polarizers and method of making same

ABSTRACT

A method for making a polarizing glass article is provided. The method includes first providing a precursor glass containing metal-halide particles. The precursor glass may be encased in a gas-permeable medium. Then, form at least a first polarizing layer and a non-polarizing region in the precursor glass. Bond the polarizing layer to a substrate and removing the non-polarizing region to expose the polarizing layer. Then, separate the first polarizing layer from the substrate to produce an ultra-thin polarizing glass article measuring less than or equal to about 200 μm in thickness. The method may further comprise cutting the polarizing layer into wafers.

This application claims the benefit of provisional application Ser. No.60/085,464 filed May 14, 1998.

FIELD OF THE INVENTION

The invention relates to ultra-thin light polarizing glass articles anda method for making same. In particular, it relates to a novel methodfor making silver-containing polarizing glass, and ultra-thin polarizingglass articles made from such glass. Specifically, the invention relatesto a single layer light polarizing device free of adjacentnon-polarizing regions.

BACKGROUND OF THE INVENTION

The use of very thin polarizing devices makes it possible to produceoptical components without the need for expensive lens elements and moreimportantly, lessen alignment problems in order to maintain high lightthroughput. For any integrated device, the goal is to shorten theoptical pathlength in order to decrease diffraction losses. These, andother aspects of vertical integration technology, as it is known, aredescribed by Shiraishi et al., in Vertical Integration Technology forFiber-Optic Circuit, OPTOELECTRONICS, Vol. 10, No. 1, pp.55-74, Mar.1995.

The closest product of which we are aware, for producing opticalisolators of the kind described herein is POLARCOR™, a high qualityfinished optical component available from Corning Incorporated, Corning,NY. This product is available in planar shapes with dimension up to 30mm parallel to the major principal transmission direction.

POLARCOR™ products are also available in thickness as low as 0.2 mm.

In addition to POLARCOR™. we are aware of a several patents such as U.S.Pat. Nos. 5,430,573; 5,322,819; 5,300,465; 5,281,562; 5,275,979;5,045,509; 4,792,535; 4,479,819; JP 4-279337; JP 5-208844; and EP 0 719741 all of which have described glass articles which are polarizing inthe infrared region.

JP 4-279337; JP 5-208844 describe a copper-based polarizing glass which,according to the patent application can measure less than 240 μm inthickness. However, the examples were limited to glass thickness in therange of 100 to 1000 μm. The polarizing glass of this patent applicationwas prepared by polishing stretched copper halide-containing glass to athickness in the range of 1000 to 1000 μm, and then subjecting the glassto a hydrogen mosphere to form glass.

One disadvantage of the methods described in the above references, atleast with respect to making ultra-thin polarizing glass, is thedifficulty of handling and processing very thin free-standing glasspieces as required in the instant invention. In this connection,JP[Hei]9-86956 suggests a method for reducing loss due to the highstress used for stretching small crystals embedded in a glass matrix.However, there continues to be a need for improvements in the processesfor making thin glass polarizers.

Accordingly, it is the object of the present invention to provideapproaches for making ultra-thin polarizing glass articles in which theabove difficulties are reduced or eliminated.

SUMMARY OF THE INVENTION

The invention relates to an ultra-thin polarizing glass, unique in thatthe glass is monolithic and has dispersed across its entire breadth andthickness, elongated submicroscopic metal particles. The metal particleshave a long axis such that the glass preferentially absorbs polarizingcomponents of light that is parallel to the long axis to allow hightransmittance of light, which vibrates perpendicular to the long axis.The polarizing glass is also unique in that it is essentially free ofmetal halide particles which tend to impart certain undesirable opticalproperties to the glass such as photochromic properties, or lightscattering caused by the presence of tiny halide crystals embedded inthe glass. In another aspect, the invention relates to a method ofmaking ultra-thin polarizing glass.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a POLARCOR™ glass which can be used toproduce ultra-thin polarizing glass;

FIG. 2 is a schematic diagram of one embodiment of the inventive methodof producing ultra-thin polarizing glass;

FIG. 3 is a schematic diagram illustrating another embodiment of theinventive method of producing ultra-thin polarizing glass: and

FIG. 4 is a schematic diagram illustrating an embodiment in which metalhalide containing glass is processed while encased in an outer skinglass.

DETAILED DESCRIPTION OF THE INVENTION

In order to produce polarizing glass articles of the invention measuringless than 200 μm in thickness, we have developed a process by whichpolarizing glass can be thinned down to form an ultra-thin polarizingglass measuring even less than 50 μm in thickness, preferably having athickness in the range of 10 to 40 μm.

In one particularly useful embodiment, the process begins with a pieceof POLARCOR™ glass 8 (FIG. 1), having two polarizing glass layers 4,separated by a non-polarizing region 6. The polarizing layers 4 of theglass contain stretched or elongated metal particles, while thenonpolarizing central region contains elongated or stretched metalhalide particles. In this embodiment (FIG. 2), to form very thinpolarizing glass articles from the POLARCOR™ glass, the glass 8 is firstblocked or bonded to a suitable substrate 10 using an appropriatematerial such as a resin, wax, or a resin/wax mixture. The glass isbonded onto the substrate such that one of the polarizing layers 4 is inabutting contact with the substrate 10. Two cover slips 12 are thenplaced on either sides of the glass as shown in FIG. 2. The slips, whichare optional, serve to protect the edges of the a Am A POLARCOR™ glassduring the lapping process.

Other appropriate blocking (bonding) compounds can also be used such asLoctite 412, a cyanoacrylate adhesive available from LocktiteCorporation, Rocky Hill, Conn. After bonding the glass to the substrate,the top polarizing layer and the central non-polarizing region areremoved, for example, by a single-side lapping process, optionallyfollowed by a polish with a cerium oxide slurry. Then, while the bottompolarizing layer is still bonded to the substrate, the glass is diced(sliced) to obtain thin wafers of polarizing glass having the desiredthickness. This dicing step can be done using a high speed precisionwafer saw. Optionally, the wafers are further cut or diced to variouslengths to obtain desired dimensions. The wafers are then separated andremoved from the substrate by dissolving the blocking compound in asuitable solvent such as acetone, optionally finished by polishing, andthen cleaned. The step of removing the top polarizing layer and thecentral nonpolarizing layer can also be done by single-side lapping,chemical thinning, or by a combination of the single-side lappingfollowed by chemical thinning. Examples of useful chemical thinningtechniques include use of suitable chemicals such as acid fluorides ormolten sodium hydroxide baths. Using this technique 1 mm ×2mm wafershaving a thickness in the range of 1 to 200 μm (preferably less than 100μm) can be obtained.

In another embodiment (FIG. 3), the POLARCOR™ glass 8 to be thinned isencased in an appropriate medium 14 such as an outer skin glass of thetype described in U.S. Pat. No. 4,486,213. In this embodiment, whilestill encased in the skin glass or other appropriate medium, thin wafersof polarizing glass are then sliced from the two polarizing layers asdescribed above. The advantage of this approach is that the outer skinglass provides additional bulk and stiffness to the piece for ease ofhandling during the thinning process. The outer skin glass issubsequently removed by any suitable method such as by chemicaldissolution.

In another embodiment, glass containing elongated metal halideparticles, preferably silver halide particles, is thinned to a desiredthickness, preferably in the range of 10 to 50 μm using any of the abovedicing and thinning techniques. After dicing and thinning to a desiredthickness, the glass is subjected to a reducing gas environment asdisclosed in U.S. Pat. Nos. 4,479,819 and 4,908,054, to convert all thesilver halide particles to elongated metallic silver particles and tothereby render the glass polarizing.

In still another embodiment, metal halide-containing glass is firstencased in an appropriate medium to form a composite structure. Themedium is preferably any gas permeable material which will not reactwith the glass. A particularly useful example of such a medium is theouter skin glass described in U.S. Pat. No. 4,486,213. The compositestructure (i.e., skin glass and metal halide-containing glass) is thenco-stretched to form a thin metal halide-containing glass in which themetal halide particles are elongated to a desired aspect ratio, forexample, as described in the U.S. Pat. No. 4,486,213 (hereinincorporated by reference). FIG. 4 illustrates one practical method ofstretching the metal halide particles. As shown in FIG. 4, the compositestructure 16 is shown comprising the metal halide containing glass 18and a skin glass 20. The composite structure 16 passes through a hotzone of a redraw furnace represented by arrow 22, wherein the compositestructure is heated and stretched under tension by pulling rolls 24 toform a stretched composite structure 26.

The stretched structure is then exposed to a reducing gas atmosphere toreduce the elongated metal halide particles and thereby form glasscontaining elongated metal particles. The structure can then be slicedas described herein to form very thin slices of polarizing glass 8encased in skin glass. The skin glass can be removed by any appropriatemethod to expose the stretched polarizing glass. For example, the skinglass can be washed away using a suitable solvent.

In this embodiment, the metal halide containing glass can be encased inthe skin glass either after it has been formed and hardened, or inmolten form while both the skin glass and the metal halide containingglass are being discharged from a separate glass feeders such asdescribed in U.S. Pat. No. 3,582,306, or laminated glass sheet formersas described in U.S. Pat. No. 4,204,027 and 4,214,886.

In still a further embodiment, metal halide-containing glass is firststretched to form elongated metal-halide particles having desired aspectratios prior to encasement in a skin glass. After encasement, thestructure is then exposed to a reducing gas environment to reduce themetal halide particles and form polarizing glass containing elongatedmetal particles.

After the reducing step the glass containing elongated metal particlescan then be thinned into desired a thickness using methods describedherein.

EXAMPLES

The following example describes one preferred method for makingpolarizing glass having a thickness less than 100 μm, preferably, lessthan 50 μm, and most preferably, in the range of 10 to 40m. ThePOLARCOR™ glass article used in the following example can be either atwo-layer (a polarizing and a non-polarizing layer) or three-layer glass(a polarizing layer sandwiched between two non-polarizing layers).

-   1) measure and record the thickness of a glass slide and a POLARCOR™    piece using a metric micrometer;-   2) place the glass slide, POLARCOR™ piece, and cover slips on a hot    plate pre-set at a low temperature to allow the pieces to warm up;-   3) smear some thermal set “rosin” on the glass slide and allow the    rosin to melt; when melted, place the glass piece of POLARCOR™ on    the rosin, place a cover slip on each side of the piece (Figure),    and remove the slide from the hot plate to allow the glass to cool;    and-   4) measure the thickness of the glass slide and POLARCOR™ piece    together, and subtract from this the initial- thickness of the glass    slide and

POLARCOR™, to obtain the thickness of the rosin (smear) in order todetermine the thickness of the POLARCOR™ during the thinning process;

-   5) after the slide has cooled sufficiently for further processing,    place the slide on a thinning apparatus such as a Buehler Petro-thin    unit (available from Buehler, Ltd., Lake Bluff, Ill.), and remove by    grinding for example, the top portions of the POLARCOR™ piece until    about 100 micron thickness is remaining;-   6) remove the slide from the unit and measure the thickness of the    slide and POLARCOR™ piece using a micrometer; from this number    subtract the thickness of the slide and the thickness of the rosin    later to obtain the amount of POLARCOR™ material remaining;-   7) grind down the POLARCOR™ to remove enough of the top portions of    the piece to obtain a very thin piece of material measuring about    20-50 μm. The grind and measure steps are repeated until the desired    thickness is obtained.-   8) Polish to obtain an ultra-thin polarizing glass piece. Any known    polishing medium can be used for this process such as a cerium oxide    polishing wheel.

In other experiments, using the methods described above, we haveproduced several batches of polarizing glass articles having a meanthickness as low as 10 μm after polishing.

To evaluate the optical performance of the glass polarizers of theinvention, several samples 15×15 mm square, having thicknesses in therange of 27 to 34 μm as measured with an optical microscope, were testedby measuring the transmission and contrast ratio of the wafers. With thewafers still mounted on the substrate, measured transmission was about90%, while contrast ratio was in the range of 3124 to 3514, at awavelength of 1510-1590 nm. Extinction ratios measured for these samplesranged between 29 and 31 dB.

As contemplated herein, the grinding step can be accomplished in onestep, or in a series of steps. For example, an initial grinding step canbe done using a 1200 grit grinding wheel in order to reduce thethickness to within a first approximation of the desired thickness,followed by a second grinding step using a 2400 grit grinding wheel toobtain the final desired thickness to within the actual specifications.

In the above illustration, it will be observed that one of the originalsurfaces of the POLARCOR™ piece, the side that is in contact with theslide, has been preserved throughout the entire grinding process. Thefinal step in producing an ultra-thin polarizing glass is to disengagethe polarizing glass piece from the slide using any known process. Wehave found the use of a solvent (e.g., methylene chloride) in andultrasonic bath to be particularly useful for this purpose.

It should be understood that the foregoing represents illustrativeembodiments of the invention, and is not intended to embody all aspectsof the invention. In addition to the above embodiments, it will be clearto persons skilled in the art that numerous modifications and changescan be made to the illustrative embodiments without departing from theintended spirit and scope of the invention.

1. A method of making an ultra-thin polarizing glass article, the methodcomprises the steps of: a) providing a precursor glass comprising afirst polarizing layer containing elongated metal particles, and anon-polarizing region containing metal-halide particles; b) bonding saidfirst polarizing layer to a substrate; c) removing said non-polarizingregion to expose said first polarizing layer; and, d) separating saidfirst polarizing layer from said substrate to form a polarizing glassarticle measuring less than or equal to about 200 μm in thickness. 2.The method according to claim 1, wherein a single-sided lapping processis used to remove said non-polarizing region.
 3. The method according toclaim 1, wherein a combination comprising a single-sided lapping processfollowed by a chemical thinning process is used to remove saidnon-polarizing region.
 4. The method according to claim 1, furthercomprising polishing said first polarizing layer.
 5. The methodaccording to claim 1, further comprising cutting said first polarizinglayer into wafers.
 6. The method according to claim 5, wherein saidcutting step is prior to separating said first polarizing layer fromsaid substrate.
 7. The method according to claim 1, wherein said firstpolarizing layer is essentially free of metal-halide particles.
 8. Themethod according to claim 1, wherein said polarizing glass article has athickness in the range of about 1 μm to about 100 μm.
 9. The methodaccording to claim 1, wherein said polarizing glass article has athickness in the range of about 5 μm to about 50 μm.
 10. The methodaccording to claim 1, wherein said precursor glass further comprises asecond polarizing layer, such that said non-polarizing region issandwiched between said first and second polarizing layers.
 11. Themethod according to claim 10, wherein after bonding said precursor glassto said substrate, both said second polarizing layer and saidnon-polarizing region are removed to expose said first polarizing layer.12. The method according to claim 11, wherein both said first and secondpolarizing regions contain elongated metal particles.
 13. A The methodaccording to claim 1, wherein said elongated metal particles have a longaxis, characterized in that said elongated metal particlespreferentially absorb a polarized component of light that is parallel tosaid long axis, and allows transmission of light that is perpendicularto said long axis.
 14. The method according to claim 1, wherein saidmetal particles include silver.
 15. The method according to claim 1,wherein said precursor glass contains elongated silver halide particlesand was subject to a hydrogen environment to reduce at least a portionof said silver halide particles to metal particles.
 16. A polarizingglass article made according to the method of claim 1,
 17. Thepolarizing glass article according to claim 16, wherein said polarizingglass article is characterized as exhibiting an extinction ratio greaterthan 25 dB in a wavelength greater than about 1500 μm.
 18. A method ofmaking a polarizing glass, the method comprising: a) performing the stepof either (i) encasing a metal halide-containing precursor glass in agas-permeable medium to form a composite structure followed by (ii)stretching the composite structure to elongate said metal halideparticles, or vice versa; b) exposing the stretched composite structureto a reducing gas atmosphere to reduce at least a portion of theelongated metal halide particles to metal particles to form a polarizinglayer; c) slicing said composite structure into wafers of medium encasedpolarizing glass having said polarizing layer and a non-polarizingregion attached thereto; d) bonding said polarizing layer to asubstrate; e) removing said non-polarizing region to expose saidpolarizing layer; and, f) separating said polarizing layer from saidsubstrate to form a polarizing glass article measuring less than orequal to about 200 μm in thickness.
 19. The method according to claim18, wherein said removing step (e) comprises grinding away saidnon-polarizing region to a predetermined thickness.
 20. The methodaccording to claim 18, further comprising removing said gas permeablemedium.
 21. A The method according to claim 18, further comprisingpolishing said polarizing glass article.
 22. The method according toclaim 18, wherein said polarizing glass article has a thickness in therange of about 1 μm to about 100 μm.
 23. The method according to claim18, wherein said polarizing glass article has a thickness in the rangeof about 5 μm to about 50 μm.